An image projection apparatus (projector) is a device that displays an image or a video by projecting the image or the video on a large screen or the like. In the image projection apparatus, when projecting a projected image on the projection surface, it is required to perform projection with highly accurate focusing.
A technique relating to adjustment of a focal range of the image projection apparatus is disclosed in, for example, Patent Document 1. Patent Document 1 discloses a technique where a phase modulation element (Imaging Lens with EDF 332) that modulates a phase of a light flux is arranged in a projection optical system, a function of allowing a change of a point image along an optical axis direction to be smaller than that of a state where there is no phase modulation element within a certain distance from the focus plane is provided, and by performing image processing such as cancelling a modulation component by the phase modulation element on the image to be projected in advance (Image Coder 314), a focusing range of the projected image is allowed to be larger than a general method.